A numerical modeling approach is being used to quantify groundwater conditions and land subsidence spatially. In addition, selected management alternatives for controlling land subsidence will be evaluated. The numerical modeling will consist of four main tasks:
The focus of the model will be the role of the fine-grained deposits, both the thick and the thinner interbedded aquitards, in aquifer-system compaction. The simulation of aquifer-system compaction is based on the established theory of aquitard drainage (Helm, 1975) and will be used in 1-D simulations of coupled groundwater flow and skeletal deformation based on the elastic and inelastic compressibilities of the aquifers and the aquitards (Hoffmann and others, 2003). The coupled model solves for hydraulic head and vertical displacement for specified aquifer-system properties as a function of depth and time. The model will permit a detailed analysis of the processes controlling compaction and also will be used to evaluate sub-regional model construction, particularly the vertical discretization of the Corcoran Clay.
The CVHM will be used to predict the effects of an extended drought on water levels and land subsidence along the DMC. The model will provide useful insight into the relation between conveyance along the Canal, land subsidence, and the water supply and demand components of agriculture and urban water uses. Simple hydrologic assumptions of drought conditions and reduced deliveries will be used to help forecast the anticipated location and magnitude of land subsidence along the Canal and surrounding area. Additional model scenarios will be completed to simulate the effects of reduced surface-water deliveries, in addition to the regional drought. The results of these scenarios will be analyzed for water levels and land-subsidence data for the simulation period.
Update, rediscretize, recalibrate, and add additional details to the CVHM with direct reference to the area of interest and (or) the ability to simulate subsidence in more spatial detail. These enhancements will provide more detailed answers to the cause and effect relations of groundwater pumping and land subsidence. Because the CVHM is a regional model developed for the Central Valley as a whole, it is spatially too coarse to be used to evaluate site-specific and sub-regional flow and subsidence conditions. The CVHM will be updated and refined in the study area to provide an improved understanding of the relationship between groundwater pumping and land subsidence. The San Joaquin Valley and Delta portions of the CVHM will be spatially rediscretized. The updated and refined model will be used to simulate the drought scenarios described in Task 2. The updated model will more accurately forecast the anticipated locations and magnitudes of land subsidence. The model results will be analyzed relative to the water use, land subsidence, and Canal structures for this period. The effects on the Canal will include the impacts of land subsidence on changes in land-surface elevations that affect Canal gradients and freeboard capacity. .
The updated/rediscretized CVHM will be used to address adaptation and mitigation alternatives to changing climate and reduced surface-water deliveries. The task will analyze the effect of changing water demand and supply on various subregions along the Delta-Mendota Canal. The USGS will work with the San Luis and Delta-Mendota Water Authority to develop potential water-management alternatives to evaluate with the CVHM.